Linear actuator system with reversing means

ABSTRACT

A linear actuator system with reversing means including a housing with a lateral opening to receive a rotating shaft that passes through the opening. A plurality of wheels are disposed against the shaft for initiating and controlling linear movement of the housing with respect to the rotating shaft, with the wheels being resiliently urged against the shaft. When the housing approaches either end of the shaft, it is caused to reverse and move in the opposite direction by means of contact with one or more studs located on wires that pass through the housing with stud contact causing a control member to be pivoted and thereby reverse the direction of the wheels. This in turn causes a reversal of movement of the housing. Friction balls are also provided on another wire that passes through the housing with the friction balls assuring a satisfactory reversal of the control member.

United States Patent [191 Zaruba et al.

[ 1 3,709,047 1 Jan. 9, 1973 [54] LINEAR ACTUATOR SYSTEM WITH REVERSINGMEANS [73] Assignee: Textol Systems, Inc., Carlstadt, NJ.

[22] Filed: Jan. 21, 1971 [21] Appl. No.: 108,490

Related U.S. Application Data [63] Continuation-impart of Ser. No.50,620, June 29,

3,434,357 3/1969 Roantree ..74/25 Primary Examiner-Milton KaufmanAssistant Examiner-Wesley S. Ratliff, Jr. Attorney-Caesar, Rivise,Bernstein & Cohen [5 7] ABSTRACT A linear actuator system with reversingmeans including a housing with a lateral opening to receive a rotatingshaft that passes through the opening. A plurality of wheels aredisposed against the shaft for initiating and controlling linearmovement of the housing with respect to the rotating shaft, with thewheels being resiliently urged against the shaft. When the housingapproaches either end of the shaft, it is caused to reverse and move inthe opposite direction by means of contact with one or more studslocated on wires that pass through the housing with stud contact causinga control member to be pivoted and thereby reverse the direction of thewheels. This in turn causes a reversal of movement of the housing.Friction balls are also provided on another wire that passes through thehousing with the friction balls assuring a satisfactory reversal of thecontrol member.

10 Claims, 12 Drawing Figures PATENTEDJAN 91975 3.70904! SHEET 1 [IF 3FIG. 7

FIG. I

ENTORS. L ZARUBA INV WENZE ATTORNEYS JAN SEHNAL PATENTEDJAH 9 I975 3,709,047

sum 2 UF 3 ATTORNEYS PATENTEDJAH ems 3.709.047

SHEET 3 OF 3 v IO FIG. ll

///1 :06 Li 32 m FIG. I0

ENTORS. WE L ZARUBA BY JAN SEHNAL Como/L, @W 9 ATTORNEYS LINEAR ACTUATORSYSTEM WITH REVERSING MEANS This application is a continuation-in-partapplication based on application Ser. No. 50,620, filed June 29, 1970,entitled Reciprocating Linear Actuator.

This invention relates to a linear actuator system with reversing meansand has as its objective the provision of a new and improved device ofthis general class.

Until recently, linear actuators were provided with a screw drive sothat rotational movement would be used to initiate linear movement.

However, the shortcomings of a screw drive were long recognized. First,the ratio of linear displacement to angular displacement could bechanged only be securing a screw drive having threads of a differentpitch. Second, the thrust of the shaft will remain constant for the rateof rotation of the shaft. Also, the direction of linear movement can bechanged only by changing the direction of shaft rotation.

The foregoing difficulties were overcome by devices shown in both ZarubaU.S. Pat. No. 3,473,393 and Steibel U.S. Pat. No. 3,475,972.

In the Zaruba Patent there was provided a linear actuator whichcomprised a plurality of wheels in contact with a rotating shaft, withthe pitch of the wheels being adjustable and changeable in order tochange the rate of movement and direction of linear displacement of therotating shaft.

In Zaruba co-pending application Ser. No. 851,421, filed Aug. 19, 1969now U.S. Pat. No. 3,638,504, there were introduced several improvementsincluding a pressure controlled diaphragm by which the pressure of thewheels against the shaft could be varied from a remote source, and thusthe thrust of the linear actuator could be varied from a remote source.

With the principle of the modern linear actuator now established, it hasbecome important to develop economies in the device in order to hastenits use in industry.

In view of the foregoing, co-pending application Ser. No. 50,620disclosed and claimed a reciprocating linear actuator having a housingreceiving a tubular member having a lateral opening with a shaft passingthrough the opening. A plurality of wheels were also mounted in thehousing with the wheels being disposed against the shaft for initiatingand controlling linear movement of the housing with respect to the shaftthat is rotated by the source of power. The housing was made of aspecial sheet metal construction, and the wheels were urged against theshaft by a single spring in a special novel arrangement.

In use, the reciprocating linear actuator could be made a part of acarriage assembly of a therapeutic massaging chair, and could be fittedwith shock absorbers so that reversal of movement for the chair can beachieved in a silent manner.

In said application Ser. No. 50,620, a spring was utilized to draw acontrol member to a reversed position utilizing an over-the-centeraction to reverse the pitch of the wheels in order to reverse themovement of the housing. While the device of said application Ser. No.50,620 has proved to be quite satisfactory, nevertheless, it has becomedesirable to eliminate the spring associated with the control membersince it is believed that over a period of time the spring will wearout.

Further attention is called to co-pending Zaruba application Ser. No.50,621, filed June 29, 1970, entitled Linear Actuator with Yoke ActuatedPitch Control." In this application there was disclosed a linearactuator with yoke actuated pitch control having a housing for receivinga plurality of yokes each including a wheel, the housing further havingan opening to permit the passage of a rotatable power driven shaft witheach of said wheels being resiliently urged against said shaft and meansbeing provided to vary the pitch of the wheels with respect to saidshaft. Each of the pitch varying means were interconnected to a controlshaft that extends from the housing. The control shaft was turned bymovement of a lever connected to the control shaft which afforded arelatively large mechanical advantage. The pressure of the wheelsagainst the power shaft was changed by means of fluid pressure. Theentire device was contained in an impervious housing.

In view of the foregoing, it is an object of the present invention toprovide a linear actuated system with reversing means which can bemanufactured at a cost that is competitive with similar devices thathave already been proposed.

Yet another object of the present invention is to provide a linearactuator system with reversing means in which the pitch of the wheelscan be varied or the direction of the linear actuator reversed while thedevice is in use.

Still another object of the present invention is to provide a linearactuator system with reversing means which eliminates the necessity toutilize a spring member in connection with the control member that givesrise to a reversing action.

Still another object of the present invention is to provide a linearactuator having pairs of opposing wheels to increase thrust and providea more positive switching action.

The foregoing as well as other objects of the invention are achieved byproviding a linear actuator system with reversing means including ahousing that has a lateral opening to receive a rotating shaft'andwherein a plurality of wheels are resiliently urged against the shaft.The wheels initiateand control linear movement of the housing withrespect to the rotating shaft. As the housing approaches either "end ofthe shaft, a control member causes the housing to reverse and move it inthe opposite direction. This occurs by contact between the controlmember and a stud that is located on a wire that passes through thehousing. Friction'balls are provided on another wire that pass throughthe housing in order to assure a satisfactory reversal'of the controlmember. The pitch of the wheels is changed in ac cordance with themovement of the control member.

Other objects and many of the attendant advantages of this inventionwill be readily appreciated as, the same becomes better understood byreference to the following detailed description when considered inconnection with the accompanying drawings wherein:

FIG. 1 is a perspective view of a linear actuator system with reversingmeans constituting an embodiment of the present invention;

FIGS. 2, 3 and 4 are plan views of various positions of the linearactuator along the rotating shaft in order to illustrate the reversingaction;

FIG. 5 is a sectional view taken along the lines 5-5 of FIG. 1;

FIG. 6 is an enlarged elevational view showing the friction ring thatextends from the control member when the friction ring is in closeproximity with a friction ball;

FIG. 7 is a perspective view of a yoke which holds the shaftcontactingwheel;

FIG. 8 is a view similar to FIG. 7 but showing another type of yoke thatisused in the embodiment of FIG.

FIG. 9 is a perspective view of an alternate housing with the yoke andcertain other portions of the actuator assembly removed for the sake ofclarity;

FIG. 10 is a sectional view taken along the lines 10- 10 of FIG. 11;

FIG. .11 is a side elevational view of another embodiment of the presentinvention; and

FIG. 12 is a sectional view showing yet another type of yoke which usesball bearings between the lowermost flange of the yoke and the flangeextending from the inner housing of the linear actuator.

Referring now in greater detail to the various figures of the drawingswherein like reference characters refer to like parts, there is shown atin FIG. 1 a linearactuator system with reversing means constituting afirst embodiment of the present invention. The linear actuator 20basically comprises a rectangular housing 22 having a transverselyextending cylindrical tube 24 (FIG. 5).

Tube 24 is continuous with the exception of centrally disposed openings26 on opposed sides of the tube 24, with the openings 26 accommodatingrotating shaft 28.

As further seen in FIG. 5, yoke assemblies 30 (such as shown in FIG. 7)are maintained within tube 24 by means of plates 32. The plates 32 aredrawn together by means of bolts 34 which extend through appropriateopenings in housing 22.

As seen in FIG. 7, the yokes 30-include stems 31 that are received inthe plates 32 (FIG. 1). Wheels 33 are revolvably mounted within slot 35in yokes 30 by means of connectors 37. As further seen in FIG. 7, eachof the yokes 30 also includes a control pin 36 that is usedto controlthe pitch of the wheel 33 within the yoke 30.

With furtherreference to FIG. 1, it will be seen that a rectangularcontrol member 38 is provided at the top of housing 22, with the controlmember 38 being pivotally connected to the housing 22 by rivet 40. Itwill be seen in FIG. 5 that the pivotal securement of the plates 38 tothe top of housing 22 also includes a pair of bearing surfaces 41 inorder to minimize the effects of wear so that the linear actuator willhave a long life.

It willbe seen from FIGS. 1 and 5 that the control member possesses attwo of its corners, U-shaped corner portions 42, each ofwhich include apair of arms 43 which rotatably embrace one of the control pins 36.Thus, rotation of control member 38 is automatically conveyed to thecontrol pins 36 which in turn will change the path of the wheels 33.

As further seen in FIG. 1, the other two corners of the control member38 possess projections 44, the purpose of which will be discussedhereinafter.

It is to be noted that the corner portions 42 of the control member 38possess rings 45 and 46 that are held by brackets48. The rings 45 and 46receive guide wires 60 and 62 as will be discussed hereinafter.

A cylindrical collar 47 is integrally secured to each end wall ofhousing 22. Collars 47 extend longitudinally of the shaft 28 and includea cylindrical bearing 49 (FIG. 1) which is provided on the inner surfaceof the bore of the collar. The collars 47 slidably embrace thecylindrical shaft 28 and maintain the housing in a substantially fixedposition radially with respect to the shaft.

The housing'22 also includes a pair of flanges 51 which are integralwith and extend transversely to the side walls of the housing. Theflanges 51, as will be seen hereinafter, act to enable securement ofball carriages or other loads to the linear actuator.

As. best seen in FIG. 5, the plates 32 each have an opening toaccommodate stem 31. The stem 31 of each of the yokes has telescopedthereover a needle bearing 44 and springs 57. The needle bearings 55enable the yokes 30 to rotate with respect to the plates 32. Springs 57act to enable a resilient urging of the wheels against the shaft 28 andfurther act to assure an equalization of the forces between both wheels33 and shaft 28.

It will be seen from FIG. 1 that the rotating shaft 28 is mounted on anarm that includes an L-shaped rail 50 that is secured to the woodenframeportions 52 of a massaging chair. The other end of the shaft 28 issupported by another L-shaped rail 54. Rails 50 and 54 are connected bya longitudinally extending bracket 55. Each end of shaft 28 possesses acam-like engaging member 56 that has anengaging edge 58 which extendsradially from the shaft 28.

A pair of wires 60 and 62 extend between the rails 50 and 54, with thewires 60 and 62 passing through engaging ring 45 and friction ring 46,respectively. The wire 60 is fitted with fixed studs 64 adjacent eachend thereof. The wire 62 is fitted with fixed friction balls 66 adjacenteach end thereof.

The. reversal of the control member 28is best seen from an inspection ofFIGS. 2, 3 and 4 wherein shaft 28 is rotating in a clockwise sense asindicated by arrow 61. With reference to FIG. 2, it will be seen thatthe housing 22 is moving from right to leftin the direction of arrowv 68by virtue ofthe rotation of shaft 28 and the engagement of wheels 33 inaccordance with Zaruba U.S. Pat. No. 3,473,393 and the'co-pendingapplications hereinabove specified, the disclosures of which areexpressly incorporated by reference herein.

Movement of housing 22 in the direction of arrow 68 continues until theengaging ring 45 abuts stud 64 as shown in FIG. 3. .When this occurs,further right to left movement in the direction of arrow 68 has theeffect of causing the control member 38 to pivot in a clockwise senseand this in turn changes the pitch of the wheels 33 with respect to therotating shaft 28. When the control member 38 reaches the position ofFIG. 3, the wheels 33 lie in a plane that is perpendicular to the axisof shaft 28 and the control member 38 is in the neutral position.

As the control member 38 approaches the position of FIG. 3, the cammember 56 which rotates with shaft 28 at the end of the housing will bein a position to abut one of the pins 44 extending from the controlmember 38. The edge 58 of the cam member 56 contacts the pin 44 andcarries the control member 38 past the dead center point in order toachieve a good switching action.

It should be noted that before the control member 38 was rotated fromthe position of FIG. 2 to the position of FIG. 3, that the friction ring46 had cleared friction ball 66 in moving from right to left from theposition of FIG. 2 to the position of FIG. 3. Thus, as the housing 22starts to move from left to right in FIG. 4, it is necessary for thefriction ring 46 again to clear the friction ball 66. However, therewill be some drag or friction as the ring 46 moves about the ball 66 andthis further insures that the control member 38 has been completelypivoted from the position of FIG. 2 to the position of FIG. 4.

The friction ring 46 and the friction ball 66 are best seen in FIG. 6.The internal surface of the friction ring 46 is lined with a bearingsurface, such as a well known plastic material sold under the TrademarkDELRIN" or TEFLON. The internal surface of the friction ring 46 isgenerally cylindrical, but somewhat convex with an inner bore 72 that isnarrowest at the center of the friction ring and which flares outwardlyat each direction from the center.

As further seen in FIG. 6, the friction ring 46 further includes atransversely extending slot that extends diametrically through thefriction ring 46 until the point 76 is reached. It follows that theportion of the friction ring 46 to the right of point 76 (as viewed inFIG. 6) is a unitary ring whereas the portion of the friction ring 46 tothe left of point 76 is actually presented in two halves or as a splitring. This enables the left side of the friction ring 46 to spread as itpasses over the friction ball 66. A C-spring 78 is slipped about theleft side of the friction ring 46 in order to provide a degree ofresiliency in the split ring side of the friction ring 46 so that thetwo halves will return together after the friction ring 46 has passedover the friction ball 66.

With reference again to FIG. 1, it will be seen that ball carriages 80are secured to blocks 82 which are suitably secured by fasteners 86 toflanges 51. The ball carriages 80 are uitlized to provide a massagingaction within the back of a therapeutic chair. The ball carriages 80ride on the rear surface of the backrest of a chair in a well knownmanner, with the ball carriages 80 acting to maintain the housing 22against rotation with respect to the shaft 28, and the ball carriages 80also act to guide the housing 22 along the shaft 28.

Alternate embodiments of the invention are shown in FIGS. 8 to 12inclusive. In FIG. 8, a yoke 100 is shown which is essentiallycylindrical. Yoke 100 is substantially similar to yoke 30 with theexception of the cylindrical wall provided about the yoke 30 whichresults in a flange 102 which extends beyond the slot 35 of the yoke.That is, the yoke 100 includes stem 31 and wheel 33 and is otherwisesimilar to yoke 30.

Also, yoke 100 includes control pin 36 which extends through thecylindrical outer wall of yoke 100 to the inner yoke. A pair of yokes100 are provided in the alternate housing 103.

As best seen in FIGS. 9 and 10, housing 103 includes a pair of walls 104which extend parallel to shaft 28. Walls 104 are connected together byan integral top wall 105 which also extends parallel to the shaft and apair of end walls 109 which extend transversely to the shaft and walls104 and 105. Cylindrical collars 47 are secured to each of the walls 102and extend longitudinally of shaft 28.

A pair of flanges 111 which extend transversely to walls 104 and areintegral therewith are also provided for enabling mounting of thehousing to a carriage assembly or other load. Each of the walls 104includes an annular flange 106 that extends perpendicular to the walls104 and radiallyfrom shaft 28.

As best seen in FIG. 10, when yokes are secured to the housing, theflanges 106 of walls 104 extend inwardly of the flange 102 of each yoke100 with the flanges 106 extending between the flanges 102 and wheels33. The yoke 100 are thus journalled by flanges 102 about the flanges106.

The construction of' the housing 103 is otherwise similar to theconstruction of housing 102 with a pair of openings 113 being providedin walls 104 which are aligned with respect to each other and whichaccommodate the fasteners 34 which are utilized to draw plates 32together to secure therein needle bearings 107 and spring members 108which are mounted over stems 31 of the yokes 100.

The control pins 36 are operatively engaged by control plates 38 in thesame manner as that in housing 22 of the preferred embodiment.Similarly, the control plate 38 is pivotably secured to the top wall ofthe housing via rivet 40 and also includes rings 45 and 46 which areutilized to accommodate the wires 60 and 62. The wires 60 and 62 includestuds and friction balls as in the main embodiment and are utilized toreverse the direction of the linear actuator housing as the shaft 28rotates.

A still further embodiment of a yoke is shown in FIG. 12 wherein theyoke includes a flared flange 152 that is flared so that it canaccommodate balls 154 to provide a frictionless engagement between theflange of the housing and the yoke. In both the embodiment of FIGS. 10and 12, a flange acts to abut the walls 104 of the housing after thewheels 33 have become somewhat worn. Thus, the presence of flanges 106and 152 serve to preventundue wear on the shaft which wouldoccur as aresult of the wearing down of the wheels 33 which would start to scratchthe surface of shaft 28. I 7

From all of the foregoing, it can be seen that the embodiments of theinvention as set forth hereinabove greatly minimize manufacturing cost.Furthermore, the structures of the present invention permit relativelyeasy replacement of the wheels and the yoke without having to remove theshaft 28 from its mounting on the frame of L-shaped rails 50. Instead,it is a relatively simple matter to remove the screws 34, and after thisis done, the plate 32 can be removed. The yoke is then removed fromeither tube 24 or away from the flanges of the walls 104 of housing 103in the embodiment of FIG. 10.

Furthermore, with the present invention, the tube 24 can be easilyremoved and the yoke is so constructed in FIGS. 10 and 12 as to preventscratching of the shaft 28 by unduly worn wheels.

The friction ball and studs as well as the housing and other componentsof the invention are made of metal, plastic or other materials that willoccur to those skilled in the art.

From the foregoing, it can be seen that reversal of the housing 22starts when the engaging ring 45 abuts stud 64 as shown in FIG. 3. Thishas the effect of initiating a change in the pitch of the wheels 33 withrespect to shaft 28. As the pitch of the wheels nears the neutralposition, the engaging member 56 at the end of the housing will be in aposition .to abut one of the pins 44 extending from control member 38.In particular, the edge 58 of engaging member 56 contacts a pin 44 andthis carries the control member 38 past the dead center or neutralrpointin order to achieve switching action. As the .housing nowvmoves in theopposite direction, the friction ring 46 must clear a friction ball 66.The drag or friction that is created as the ring 46 rides about thefriction ball 66 insures a complete switching action of control member38 and insures that the pitch of the wheels 33 is now in thediametrically opposite sense.

Without further elaboration, the foregoing will so fully illustrate ourinvention that others may, by applying current or future knowledge,readily adapt the same for use under various conditions of service.

What is claimed as the invention is:

1. A linear actuator system with reversing means including a housinghaving a lateral opening to receive a rotating shaft that passes throughsaid opening, the opposite ends of said" shaft being secured forrotation, a plurality of wheels associated with said housing anddisposed against said shaft for initiating and controlling linearmovement of the housing along said shaft, at least one stud located nearthe end of said-shaft, a control member secured to said housing forcontrolling the pitch of said wheels with respect to said shaft formovement of said housing along said shaft, said control member havingmeans to contact said stud as said housing approaches one of the ends ofsaid shaft, said contacting means serving to automatically startreversal of v the pitch of said wheels with respect tosaid shaft, andmeans adjacent to said shaft for engaging said control member tocontinue reversal of said pitch of said wheels to cause said housing tomove in the opposite direction.

2. The linear actuator system of claim 1 including friction ballsassuring the complete reversal of said control member.

3. The linearactuator system of claim 2 wherein said studs and frictionballs are mounted on support wires, said control member including ringsthrough which said wires pass, one of said rings being adapted to abutsaid stud and the other ring member being adapted to pass about one ofsaid friction balls in a friction action.

4. The linear actuator system of claim 3 wherein the internal surface ofsaid friction rings is somewhat convex with a plastic bearing material.

5. The linear actuator system of claim 1 including pin members extendingfrom said control member, and said means adjacent said shaft .comprisinga cam secured adjacent at least one end of said shaft, said cam beingadapted to contact at least one of said pin members to assist in themovement of said control member in order to reverse the direction ofsaid wheels.

6. A linear actuator including a housing having a lateral opening toreceive a rotating shaft that passes through said opening, said housingclosely surrounding said shaft and having a plurality of annularprojections extending radially with respect-to said shaft from saidhousing and each projection having a cylindrical bore, a plurality ofholders each having a wheel mounted therein, said holders each having acylindrical flange which is journalled over one of said fprojectionswith said wheel extending into the bore 0 said pro ection and means forresiliently urging each of said holders toward said shaft to cause saidwheels to bear against the outer periphery of siad shaft, the pitch ofsaid.

wheels being rotatable with respect to said shaft by rotation of saidholders about said projections.

7. The linear actuator of claim 6 wherein said housing includes a pairof planar walls which extend parallel to said shaft on diametricallyopposed sides thereof, each of said walls including one of said radiallyextending annular projections.

8. The linear actuator of claim 6 wherein said walls further include atleast one opening, a pair of plates, said plates being secured togetherby a fastening means extending through said openings in said walls formaintaining said holders with said wheels urged against said shaft.

9. The linear actuator of claim 6 wherein said plates are connected tosaid holders by needle bearings which act to reduce the frictionalengagement between said plate and said holder.

10. The linear actuator of claim 6 wherein said cylindrical flange ofsaid holders is flared so that ball bearings can be utilized betweensaid cylindrical flang and said annular projection.

1. A linear actuator system with reversing means including a housinghaving a lateral opening to receive a rotating shaft that passes throughsaid opening, the opposite ends of said shaft being secured forrotation, a plurality of wheels associated with said housing anddisposed against said shaft for initiating and controlling linearmovement of the housing along said shaft, at least one stud located nearthe end of said shaft, a control member secured to said housing forcontrolling the pitch of said wheels with respect to said shaft formovement of said housing along said shaft, said control member havingmeans to contact said stud as said housing approaches one of the ends ofsaid shaft, said contacting means serving to automatically startreversal of the pitch of said wheels with respect to said shaft, andmeans adjacent to said shaft for engaging said control member tocontinue reversal of said pitch of said wheels to cause said housing tomove in the opposite direction.
 2. The linear actuator system of claim 1including friction balls assuring the complete reversal of said controlmember.
 3. The linear actuator system of claim 2 wherein said studs andfriction balls are mounted on support wires, said control memberincluding rings through which said wires pass, one of said rings beingadapted to abut said stud and the other ring member being adapted topass about one of said friction balls in a friction action.
 4. Thelinear actuator system of claim 3 wherein the internal surface of saidfriction rings is somewhat convex with a plastic bearing material. 5.The linear actuator system of claim 1 including pin members extendingfrom said control member, and said means adjacent said shaft comprisinga cam secured adjacent at least one end of said shaft, said cam beingadapted to contact at least one of said pin members to assist in themovement of said control member in order to reverse the direction ofsaid wheels.
 6. A linear actuator including a housing having a lateralopening to receive a rotating shaft that passes through said opening,said housing closely surrounding said shaft and having a plurality ofannular projections extending radially with respect to said shaft fromsaid housing and each projection having a cylindrical bore, a pluralityof holders each having a wheel mounted therein, said holders each havinga cylindrical flange which is journalled over one of said projectionswith said wheel extending into the bore of said projection and means forresiliently urging each of said holders toward said shaft to cause saidwheels to bear against the outer periphery of siad shaft, the pitch ofsaid wheels being rotatable with respect to said shaft by rotation ofsaid holders about said projections.
 7. The linear actuator of claim 6wherein said housing includes a pair of planar walls which extendparallel to said shaft on diametrically opposed sides thereof, each ofsaid walls including one of said radially extending annular projections.8. The linear actuator of claim 6 wherein said walls further include atleast one opening, a pair of plates, said plates being secured togetherby a fastening means extending through said openings in said walls formaintaining said holders with said wheels urged against said shaft. 9.The linear actuator of claim 6 wherein said plates are connected to saidholders by needle bearings which act to reduce the frictional engagementbetween said plate and said holder.
 10. The linear actuator of claim 6wherein said cylindrical flange of said holders is flared so that ballbearings can be utilized between said cylindrical flange and saidannular projection.